Most rotating machines comprise a stationary member, commonly referred to as a stator, and a rotating member, known as a rotor, that rotates with respect to the stator. For many rotating machines information concerning the angular position of the rotor with respect to the stator is critical for proper machine operation. Positioning systems need such information to allow for accurate positioning of elements that are coupled to the rotor. Other types of machine systems, e.g., systems including switched reluctance machines and certain types of brushless permanent magnet machines use rotor position information to control the energization of the phase windings within the stator.
Rotor position information is typically provided by a rotor position transducer that comprises a stationary element containing one or more sensors, and a rotating element that is coupled to the shaft of the rotor such that it rotates with the rotor. The rotating element, commonly known as a "shutter," typically comprises one or more elements that interact with the sensors, such that the outputs from the sensors provide an indication of the angular position of the rotor with respect to the stator. For example, a shutter may comprise a number of outwardly extending "shutter teeth" that pass through optical sensors to provide information concerning the angular position of the rotor.
In known rotating machine systems the active portion of the rotor and the stator is enclosed in a motor housing that includes "end-shields" at the end of the housing. Such end-shields typically include a hole through which the shaft of the rotor passes and a recess that holds bearing structures that allow the rotor to smoothly rotate within the stator. In known systems a sensor board comprising a printed circuit board containing rotor position sensors is typically mounted directly to the end-shield of the motor in some fashion such that the sensors project inward into the motor housing (when the sensor board is positioned on the end-shield such that it is inside the motor housing when the end-shield is in place) or outward from the motor housing (when the sensor board is positioned on the end-shield such that it is outside the motor housing when the sensor board is in place).
Because rotor position information can be critical to proper operation, sophisticated alignment techniques are commonly employed to ensure that the sensor board is properly positioned with respect to the end-shields, that the end-shield is properly positioned with respect to the motor housing, and that the motor housing is properly positioned with respect to the stator. These complex alignment techniques increase the manufacturing costs and the manufacturing overhead associated with motors using such rotor position sensor. Moreover, these alignment techniques are not always accurate and can result in some misalignment of the sensor board with respect the stator, which may degrade the performance the motor.
Still further, the alignment techniques provide some tolerances that while individually small, may combine to significantly diminish motor performance. For example, the stator is positioned within the motor housing to a position within a first given tolerance level of a desired stator position. The sensor-board is then positioned on the end-shield within a second given tolerance of a desired sensor-board position. The end-shield is then mounted to the main motor housing within a third tolerance level of a desired end-shield position. While each of the stator, sensor board, and end-shield are positioned within tolerance, the overall positioning of the sensor board's sensors with respect to the stator may differ from the desired position by such an amount that motor performance is degraded. Thus, there is an "accumulative tolerancing" problem with such systems.
Another drawback with standard rotor position sensors where the sensor board is mounted to the end-shield involves servicing of the motors after they are in the field. For many motors there are certain elements (e.g., the bearing structures) that are necessarily within the enclosed rotor housing that may require service in the field. To reach these elements, the end-shield must be removed from the rotor housing. Because accurate positioning of the sensor board on the end-shield with respect to the motor housing is critical, the replacement of the end-shield is not a simple matter and often involves complex realignment techniques and apparatus. When such techniques are not followed, or are improperly performed by the service technician, the relative position of the sensor board with respect to the stator may change slightly, producing a misalignment that results in degraded motor performance.
Known rotor position sensors also suffer from limitation associated with the positioning of the shutter with respect to the rotor. In many systems the shutter is simply affixed to the shaft of the machine within a given tolerance level of a desired shutter position. This can further exacerbate the compounded tolerancing problem discussed above. Moreover, in systems where the shutter is mounted outside the motor housing, the shutter will typically have to be removed when servicing of elements within the motor housing is required, thus requiring realignment--and potential miss-alignment--of the shutter with the rotor upon re-assembly
It is an object of the present invention to overcome these and other limitations of known systems by providing an improved rotor position sensing system that allows for accurate positioning of the sensor elements with respect to the stator in a manner that is less sensitive to tolerance and field service problems than known systems; that is easier to manufacture than known systems; and that allows the angular position of the rotor with respect to the stator to be determined more accurately than with known systems.